This invention relates to a process for converting aqueous phosphorus wastes into leach resistant residues with provision for recovering phosphorus as high purity phosphorus compounds.
Elemental phosphorus is produced commercially by the reduction of phosphate ore in an electric furnace. In this process, the phosphate bearing ore, a carbon reductant usually coke, and a flux usually silica are charged into an electric furnace and the charge materials heated to melting temperatures. Elemental phosphorus, resulting from reduction of the phosphate by the carbon reductant, is expelled from the molten reaction mass as a vapor along with carbon monoxide plus small amounts of other gases. Ferrophos and slag are molten by-products of the reduction which are removed by tapping the furnace.
As the gaseous stream leaves the furnace, it entrains particles of carbon, phosphate ore and silica as well as other materials that may be present in the furnace. Some of these particulates are removed from the stream by passing it through a device for separating solids from a gas such as an electrostatic precipitator.
The solids-depleted stream is then led into a spray tower where the phosphorus vapor is condensed to liquid phosphorus by the cooling spray units in the tower. Liquid phosphorus is collected in the sump at the bottom of the tower where the flammable product, which is spontaneously ignitable in air, is stored under water to protect it from atmospheric exposure.
The phosphorus is pumped from the sump or a storage vessel into rail tank cars for shipment and delivery.
A commercial electric furnace phosphorus plant generates a number of aqueous waste products containing water, phosphorus and particles of entrained furnace material. The phosphorus occurs mainly as droplets suspended in the aqueous medium; very little is found dissolved since its water solubility is only 3.3 milligrams per liter.
Aqueous phosphorus wastes originate at various stages in the operation of the phosphorus plant.
For example, an aqueous phosphorus waste in the form of a slurry results when entrained furnace particles in admixture with phosphorus are removed from the electrostatic precipitator and quenched in water. The phosphorus content of the aqueous slurry generally ranges from about 0.1% to about 1.0%, typically about 0.5%; entrained particulate material from about 5% to about 20%, typically about 10%.
Another aqueous waste product associated with the operation of a phosphorus plant is created when phosphorus is recovered from phosphorus sludge. Phosphorus sludge consists of an intimate blend of phosphorus, water and entrained furnace particulates not trapped by the electrostatic precipitator. It is found in the spray tower sump and as a residue in storage tanks and railway cars.
Phosphorus is separated from the sludge by filtration or centrifugation and then combined with the plant phosphorus output or burned to make phosphorus pentoxide for conversion to phosphoric acid. The aqueous waste stream from the phosphorus recovery usually contains from about 0.1% to about 3% phosphorus, typically about 0.2 to about 0.6%.
In general, the herein aqueous phosphorus wastes are formed wherever phosphorus made by the electric furnace process and water come into contact. These phosphorus wastes, or phossy water as they are commonly referred to, vary in phosphorus assay from a few ppm to several percent.
In the disposal of aqueous phosphorus wastes from an electric phosphorus furnace, it has been the practice to place them in lined storage ponds. The solids settle to the bottom and are covered with soil to prevent spontaneous ignition of the phosphorus. Clarified water is recovered for use in the phosphorus plant.
Phosphorus and heavy metals in the entrained furnace solids are toxic substances thereby posing the risk of environmental contamination when aqueous phosphorus wastes are buried. Even plastic lined evaporation ponds can become leaky from aging or damage allowing the contents to seep into the soil.
Accordingly, phosphorus producers are phasing out retention ponds and directing their efforts to developing environmentally compatible techniques for disposing and handling such aqueous phosphorus wastes.
Although the percent of phosphorus lost to landfills or storage ponds is only a small amount of a plant's total output, it is not inconsequential, and may amount to thousands of pounds over the years. A process for recovering waste phosphorus would not only prevent environmental contamination but would increase plant output.
It is known to reduce phosphorus contaminated aqueous wastes by treating them with lime at a pH of about 9 to 10 whereby the phosphorus is converted to insoluble calcium phosphates which are recovered by settling and filtration. After contact with activated carbon to remove residual particles of insoluble phosphate, the filtrate can be discharged with no reported adverse environmental effects; see U.S. Pat. No. 4,402,833 to Bennett et al. A similar approach is proposed in U.S. Pat. No. 4,284,515 to Liu.
According to the aforecited patents, the processes disclosed therein are effective for eliminating phosphorus from aqueous waste streams in which the level of elemental phosphorus is of the order of 10 ppm or less. Nothing is said about dealing with high level aqueous phosphorus wastes.
It is also known to reduce the heavy metal content of wastes to below toxic concentrations. For instance, U.S. Pat. No. 4,950,409 to Stanforth discloses reacting solid wastes containing cadmium with a combination of lime and carbon dioxide. The cadmium is lowered from 0.5 to 3.2 ppm in the feed material to a non-leachable level of &lt;1 ppm. In U.S. Pat. No. 4,671,882 to Deere, an aqueous waste containing 40 ppm of cadmium is treated sequentially with a phosphate at low pH, a coagulant and a calcium compound to raise the pH above 7.0, and then dewatered. In U.S. Pat. No. 4,737,356 to Wheelabrator, fly ash is mixed with a soluble phosphate and lime to reduce the cadmium concentration below 1 ppm. U.S. Pat. No. 5,037,479 to RMT discloses treating solid waste having a cadmium assay of 0.06 to 13 ppm with magnesium oxide plus triple superphosphate whereby the cadmium is rendered non-leachable. In U.S. Pat. No. 4,434,060 to GE, cadmium in aqueous wastes is insolubilized by treatment with lime.
It is to be noted that in these processes, the heavy metal wastes and phosphorus wastes are treated in separate operations. Whether the processes would be effective in dealing with aqueous wastes containing both phosphorus and heavy metal contaminants is a matter of conjecture.
It is to be further noted that the cadmium and phosphorus content of the aqueous wastes treated in the prior art processes are quite low--less than about 10 ppm of phosphorus and 40 ppm of cadmium.
By way of contrast, aqueous phosphorus wastes generated in a phosphorus plant commonly contain up to several percent of free phosphorus plus a high heavy metal content, with cadmium assays often exceeding 1% (10,000 ppm). Apparently such high level wastes would need to be greatly diluted before being amenable to treatment by the prior art processes.
With the imminent banning by governmental regulatory agencies of storage ponds for containment of aqueous wastes associated with phosphorus manufacture, there is created an urgent need to develop environmentally safe means for handling and disposing of these toxic materials.